1. Field of the Invention
The present invention relates to devices and methods for generating ions. More specifically, the invention relates to compact devices and methods for generating ions using a corona discharge at or near atmospheric pressure.
2. Description of the Related Art
Radioactive isotopes such as 241Am or 63Ni are commonly used as ionization sources to generate ions in a surrounding gas stream. Radioactive ionization sources have the advantage of simplicity, compactness, durability, and reliability. The regulations associated with these radioactive ionization sources, however, may render the incorporation of radioactive isotopes into a product economically unfeasible.
Electric field ionization has the advantage of simple design, relatively simple fabrication, and low power consumption. In electric field ionization, a large electric field between 107 to 108 V/m is generated between two electrodes. The large electric field accelerates any ions within the field thereby causing the accelerated ions to collide with surrounding gas molecules. The collision of an accelerated ion and a gas molecule creates an ionized molecule.
A corona discharge is a type of electric field ionization where a neutral fluid such as, for example, air is ionized near an electrode having a high electric potential gradient. Such a potential gradient is achieved by using a discharge electrode, having a small radius of curvature. The polarity of the discharge electrode determines whether the corona is a positive or negative corona. The corona has a plasma region and a unipolar region. In the plasma region, electrons avalanche to create more electron/ion pairs. In the unipolar region, the slowly moving massive (relative to the electron mass) ions move to the passive electrode, which is usually grounded. If the plasma region grows to encompass the passive electrode, a momentary spark or a continuous arc may occur. The spark or arc may damage the electrodes, produce contaminant ions, and reduce the lifetime of the ionization source. Therefore, there remains a need for devices and methods for compact ionization sources with longer lifetimes.